1. Field of the Invention
The present invention relates to a method for detecting a defect in a bottle based on an imaging technology, and more particularly to a method for detecting a refractive defect in a barrel portion of the bottle.
2. Description of the Related Art
Defects in a barrel portion of a glass bottle are classified into shading defects which block applied light to prevent the light from passing therethrough and refractive defects which refract applied light. The applicant of the present application has proposed a method of detecting both shading defects and refractive defects as disclosed in Japanese patent publication No. 6-90150. According to the disclosed method, a light shield plate (referred to as an obliquely slit plate in the specification) having a plurality of oblique slits is positioned between a source of diffused light and a bottle under test. Diffused light from the source passes through the light shield plate and the barrel portion of the bottle, producing an image of a striped pattern. The image of the striped pattern is scanned to compare the levels of brightness at three closely positioned spots in the image for flaw detection.
If the bottle suffers large wall thickness variations, then the striped pattern tends to be so irregular that it is difficult to detect defects in the barrel portion of the bottle. The inventors of the present invention have made efforts to develop a method of processing images of irregular striped patterns particularly for the detection of thin blisters among different types of blisters which are one kind of refractive defects. The thin blister refers to a blister in the form of a thin lens on a surface of the glass bottle or in the glass bottle.
Further, the inventors of the present invention have made efforts to develop a method of processing images of irregular striped patterns particularly for the detection of longitudinal streaks. The longitudinal streak refers to a linear recess that appears partly in the longitudinal direction of the bottle.